In the manufacture of polymeric foams, such as, for example, polyurethanes, polyureas, and the like, a heat activated or gaseous chemical blowing agent is normally added to a liquid resin material in order to provide the cell structure after polymerization.
The term liquid resin material is understood to include any reactive liquid material that can be converted into a polymer by a polymerization reaction. Of particular interest are polyurethane, polyurea and isocyanate polymers which are produced by contacting under reactive conditions, suitable amounts of liquid resin material comprising a polyahl and an isocyanate.
The term polyahl is understood to include any compound containing active hydrogens in the sense of the Zerewitinoff test, see Kohler, "Journal of the American Chemical Society, page 381, Volume 49 (1927). Representative active-hydrogen groups include-OH, COOH, --SH and --NHR where R is H, alkyl, aryl and the like.
The term isocyanate is understood to include organic isocyanates and polymeric derivatives thereof useful in making polyurethanes, polyureas and polyisocyanurates such as aromatic, aliphatic and cycloaliphatic polyisocyanates. Exemplary compounds include toluene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), polymeric diphenylmethane diisocyanate, hexamethylene diisocyanate and mixtures thereof.
A crude polyisocyanate may also be used in the practice of this invention, such as the crude toluene diisocyanate obtained by the phosgenation of a mixture of toluene diamines or the crude diphenylmethane diisocyanate obtained by the phosgenation of crude methylene diphenylamine. The preferred undistilled or crude polyisocyanates are disclosed in U.S. Pat. No. 3,215,652, incorporated herein by reference. Derivatives of the above identified isocyanates, such as, prepolymers, are equally suitable for use in the present invention. This disclosure relates to systems used in RIM (Reaction Injection Molding) and RRIM (Reinforced Reaction Injection Molding).
RIM and RRIM foam processing systems generally utilize an amine terminated polyahl, polyether or polyester polyahl and MDI and TDI and small amounts of catalysts and surfactants. The compressed gas bubbles present during mold filling aid in complete filling of the mold and enhancement of surface characteristics of the molded product.
Prior art has called for the dispersion or dissolution of low boiling blowing agents to expand as gas bubbles during the polymerizing process and a dispersed gas to form heterogeneous nucleation sites. This was useful in urethane systems whose heat of reaction was high enough to raise the mixture temperature above the boiling point of the blowing agent.
Due to the extremely short gel time (&lt;1.5 sec) of current RIM and RRIM systems, low boiling blowing agents are seldom used. They do not have enough time to boil and produce expansion, unless their levels are high, causing other problems. Currently, the role of nitrogen or some other inert gas or mixture thereof has become that of a blowing agent.
In RIM and RRIM technology, the introduction of nitrogen, dry air, inert gas or a mixture thereof as a blowing agent into the polyahl or isocyanate under pressure occurs in the form of suspended bubbles, in up to about a fifty percent by volume concentration as measured at atmospheric conditions. The term "nucleation" has been retained and has become synonymous for this process. Attempts have been made to introduce the gas accurately and efficiently and to reduce the specific gravity of the liquid resin material/gas mixture by this method.
The criterion for "enough nucleation", or "good nucleation" is how well the reacting materials fill the mold. Symptomatic conditions of "poor" fill and consequently "poor nucleation" are typically pin-holing in the part; flow lines in the part; underfills in the mold; high part density gradient; sink marks in the part; porosity in the part; and air bubbles in the part.
Although it can be stated that any of these problems can be caused by other variables, they are often attributed to poor nucleation.
It is easily observed that open samples of nucleated material behave with marked differences depending on operating parameters of a machine and they vary from machine to machine. The qualitative observation is that material when withdrawn from the system, rises first as a creamy froth. After some time, the bubbles begin to coalesce and the froth eventually collapses back to a liquid. Some nucleated mixtures form froths which are more or less stable than others.
Factors which influence the stability of the froth are total gas quantity and bubble size, given a constant surface tension. It is well known that traditional methods of on-line nucleation measurement yield data which indicates "enough nucleation" is present yet molded parts may show the above stated flaws; pin-holing, sink marks, etc. At such times, an open sample shows that coalescence is occurring at a very rapid rate.
Bubble size also is the primary variable influencing the total quantity of gas a mixture may retain. It is also known that finished parts made with mixtures containing finer bubbles have greater impact strength than parts made from mixtures containing larger bubbles at identical gas concentrations. It is further known that mixtures with extremely small bubbles can be made to hold more gas than mixtures with large bubbles.
Because the blowing agent is not significantly dissolved in the liquid, it's precise volume percent in the liquid is established by measuring the density of the liquid/gas mixture. Various measuring methods summarized below are currently being employed to establish the density of the monomer/gas mixture.
One of these is the open cup method of which there are three versions. In a first; a quantity of liquid resin material/gas mixture is drawn from the apparatus conditioning loop into a dome-shape, lidded cup of known volume and weight. The cup is filled, excess material is wiped from the bleed hole in the lid and the cup is weighed. The tare is subtracted and a multiplier is used to convert the weight (grams) to the specific gravity (metric density). In another, a quantity of the liquid resin material/gas mixture is drawn from the apparatus conditioning loop into an open cup, less lid. A hydrometer is placed in the mixture and the specific gravity is read directly. In a third, a quantity of the liquid resin material/gas mixture is drawn from the apparatus conditioning loop into a graduated glass cylinder. The material height in the cylinder is recorded; the cylinder is weighed and the density calculated.
Another measuring method is known as the instrumented open cup. An automated and instrumented duplication of the open cup method, the instrumented open cup, has been patented in Europe by Krauss Maffei (EP 125,541-B). This method falls short of the open cup in that it involves allowing excess material to spill from a graduated primary weighing cylinder into a secondary cylinder, under atmospheric conditions, thereby causing the material to effervesce sufficiently that the remaining monomer in the measuring cylinder will vary significantly in density.
Another measuring method is known as the compression method. This method compresses the liquid resin material/gas mixture in a cylinder to a pressure which is intended to guarantee that the gas in the mixture is of insignificant volume. The uncompressed density is then inferred from the volume change which is actually seen as linear displacement of the cylinder.
A fourth method is known as the nuclear attenuation method. The liquid resin material/gas mixture density is measured by installing a clamp-on 15-100 millicurie radiation source emanating through a window in the housing and passing through the pipe and liquid resin material/gas stream to a receiver located opposite of the source. The detector, being either a scintillation tube or an ion chamber, emits a signal compared with a calibration standard and scaled to engineering units. The unit is extremely accurate once calibrated.
In a fifth method, the vibrating U-tube method, the liquid resin material/gas mixture passes through a vibrating U-tube oscillating in a direction perpendicular to the plane of U-tube flow. The inertial forces creating an amplitude which is directly related to the density of the mixture.
The primary disadvantage of the open cup method is that it is difficult to obtain consistent and accurate readings, since gas bubbles begin to coalesce. The instant the sample is removed from the system, and the time between removing the sample from the system and taking the measurement may vary. While the compression, nuclear attenuation method, and vibrating U-tube methods referred to above may provide more consistent and accurate density readings, they have been found to be even less useful than the open cup method in predicting the quality of polymers made from the liquid resin material. The instrumented cup method and the vibrating U-tube are only accurate if they are calibrated for the particular resin and have means to correlate the results with standard temperature and pressure. Finally, a disadvantage of all of the above methods is that they are dependent upon the skill and judgment of the chemist or operator using the methods, interpreting the results, and adjusting the process for adding gases to the liquid resin material in order to optimize the physical properties of polymers made from such material.
It is noted that the patent art provides numerous examples of apparatus and methods for using various blowing agents in a variety of liquid resin material components. For example, the introduction of an inert gas, such as, nitrogen, into a liquid reaction component of a reaction injection molding (RIM) system is taught by U.S. Pat. No. 4,157,427. In general, the gas is added to one of the precursors of a polyurethane by use of a sparger through which the gas is forced, under pressure. The sparger is described as a suitably sized and shaped porous rigid structure, to produce minute bubbles for better mixing, that is placed in a pipe through which the reactive component is circulated from the supply tank and then sent either to a mixing head or back to the supply tank. However, laminar flow enhances coalescence.
U.S. Pat. No. 4,376,172 is directed toward a closed loop apparatus for controlling the addition of a gas to a liquid, such as, polyurethane precursor, in a RIM process. Additionally, means are provided for accurately measuring the amount of the gas that is added. The blowing agent or gas is added by means of a sparger which is in a stream of the reactant being recirculated from the supply tank, through a conditioning loop and back to the supply tank.
U.S. Patent No. 4,526,907 is directed toward a process and device for charging gas into at least one of the components combined to produce plastic foams. The reactant from one supply tank is piped through a circulation line which has a zone of compression that is higher in pressure than that in the supply tank. In this compression zone the foaming gas is added, and the mixture is subsequently forced through a throttle element to reduce the pressure before return to the supply tank. The patent also teaches that several different methods can be employed to determine the amount of gas in the gas-reactant mixture including density, partial pressure, the absorption of a beam of light, compressibility and solubility, but does not necessarily discuss means for doing so.
U.S. Pat. No. 4,906,672 is directed toward a method for the continuous manufacture of polyurethane foam. More particularly, it deals with the additions of small amounts of carbon dioxide to polyurethane-forming reactants which contain water as the primary blowing agent and teaches that the carbon dioxide is to be dissolved into one of the reactants well before being sent to the mixing head.
Introduction is performed under high pressure, preferably 75 to 900 psig (0.62 to 6.3 MPa), in a pipe, a sufficient distance from the mixing head that uniform entrainment is achieved upon traveling from the sight of impingement to the mixing head. Once the mixture reaches the mixing head, a nozzle or series of nozzles are employed to expand the carbon dioxide-reactant mixture; however, the patent teaches that the entrainment of bubbles is to be avoided.
Finally, European Pat. No. 125,541-B, noted hereinabove, discloses a device for measuring the gas charging of a liquid component used for producing synthetic plastic foam, such as a polyurethane. It employs a measuring vessel, for receipt of a liquid sample periodically, and which communicates with an overflow vessel. By allowing the pressure in the measuring vessel to decrease to atmospheric, the gas laden component expands and overflows to the overflow vessel which allows density to be determined.
The prior art teaches determination of gas loading using measured density of the blowing agent/liquid resin material mixture at actual operating pressure (U.S. Pat. No. 4,157,427) or at ambient pressure. To this purpose, mixtures of polyahls and blowing agents are expanded either from a preset operating pressure to a second, lower set pressure (U.S. Pat. No. 4,376,172) or from a given preset operating pressure to atmospheric pressure (European Pat. No. 125,541-B). The latter invention utilizes equipment that is large, cumbersome and expensive. Moreover, at least part of the gas in the mixture will be lost from the system during expansion.
Thus, it should be apparent that although others have employed various low boiling compounds, as blowing agents for polyurethane foam, apparatus and method have not been taught for determining and controlling the fineness of blowing agent dispersion in a liquid resin material, in precise amounts and bubble and droplet sizes so as to control the cell structure of the resulting foam.